Changing the page size for Reports is fairly straight-forward. There are “Paper Options” tabs common to all reports. The page dimensions are easily exposed for editing. And those dimensions are saved in a .gramps/report_options.xml configuration file. (But can be over-ridden with the .gramps/books.xml
The other features of Gramps that print are all views: the Geography views and only SOME of the Charts view modes. Printing from views uses the OS print dialogs … which makes it more complex to troubleshoot where persistant settings are saved.
We are lucky to have a developer (@ztlxltl ) actively beta testing their new addon Charts view mode: tentatively called “FamilyTreeView”. (The confusion you experienced with the Family Tree addon graphical report is a good reason to kick around ideas for a more unique and evocative addon name.) That new Chart view supports printing. So that developer is well-placed to explore any Print idiosyncrasies.